Bone marrow stromal cells can give rise to several mesenchymal lineages. The existence of a common stem/progenitor cell, the mesenchymal stem cell, has been proposed, but which developmental stages follow this mesenchymal multipotent progenitor is not known. Based on experimental evidence, a model of mesenchymal stem cell differentiation has been proposed in which individual lineages branch directly from the same progenitor. We have verified this model by using clonal cultures of bone marrow derived stromal fibroblasts. We have analyzed the ability of 185 non-immortalized human bone marrow stromal cell clones to differentiate into the three main lineages: osteo-, chondro- and adipogenic. All clones but one differentiated into the osteogenic lineage. About one third of the clones differentiated into all three lineages analyzed. Most clones (60-80%) displayed an osteo-chondrogenic potential. We have never observed clones with a differentiation potential limited to the osteo-adipo- or to the chondro-adipogenic phenotype, nor pure chondrogenic and adipogenic clones. How long the differentiation potential of a number of clones was maintained was assessed throughout their life span. Clones progressively lost their adipogenic and chondrogenic differentiation potential at increasing cell doublings. Our data suggest a possible model of predetermined bone marrow stromal cells differentiation where the tripotent cells can be considered as early mesenchymal progenitors that display a sequential loss of lineage potentials, generating osteochondrogenic progenitors which, in turn, give rise to osteogenic precursors.